1. Field of the Invention
This invention relates to a bi-directional scan circuit, and is particularly applicable to a horizontal deflecting circuit of bi-directional deflection.
2. Description of the Related Art
With respect to this type of deflecting circuits, there has been proposed a deflecting circuit (hereinafter referred to deflecting circuit of bi-directional deflection) in which a horizontal deflecting coil is driven by using a drive signal, the signal level of which before and after a predetermined point in time changes symmetrically with respect to such point in time as a sine wave (U.S. Pat. No. 4,672,449).
In accordance with the bi-directional deflecting circuit, it is possible to form a display image by both scanning from the left to the right on a screen (hereinafter referred to scanning of forward path) and scanning in the opposite direction thereof from the right to the left on the screen (hereinafter referred to scanning of backward path). It is also possible to reduce the deflecting frequency to 1/2.
Further, since it is possible to prevent an abrupt change in the deflecting current such as a sawtooth signal, an unnecessary radiation or the like can be reduced and a load on the deflecting circuit element can be reduced.
Specifically, if the deflecting circuit is composed of a resonance circuit to drive a deflecting coil by a sine wave as shown in FIGS. 1A and 1B, the electric power necessary for deflection can be reduced by a simple construction (U.S. Pat. No. 5,051,668).
There is a problem, however, in this type of bi-directional deflecting circuit that an AFC circuit for raster scanning is not applicable thereto.
Specifically, in a conventional horizontal deflecting circuit based on the conventional raster scanning, after generating a sawtooth signal from flyback pulse, the result of phase comparison between such sawtooth signal and the horizontal synchronizing signal is obtained by a phase comparator, the phase comparison result being fed back to a horizontal oscillation circuit.
Thereby, in the conventional horizontal deflecting circuit, the horizontal oscillation frequency is controlled on the basis of the horizontal synchronizing signal to form an AFC circuit.
On the other hand, in a bi-directional deflecting circuit, display images are formed by forward path and backward path. Thus, even a simple shift in the phase of horizontal deflection results in the display images of forward path and backward path which are displayed at different positions.
In this case, in raster scanning, while the display position of the display image is simply shifted on displaying, the display image in bi-directional deflection is displayed as an overlapped image or, if the shift is relatively smaller, resolution in the horizontal direction is degraded.
Further, in raster scanning, picture quality of the display image is not specifically affected even if retrace time is changed due to variance in circuit elements. On the other hand, in bi-directional deflection, the display images of forward path and backward path are displayed in different positions also in this case, resulting in degradation of picture quality of the display image.
In view of the above points, the present invention has been made to propose a deflecting circuit which can be suitably used in bi-directional deflection to form a display image in synchronization with a horizontal synchronizing signal and to previously prevent degradation in picture quality of the display image.
In addition, in a conventional AFC circuit, a control voltage for controlling the horizontal oscillation frequency is generated at the phase comparator to drive the horizontal deflecting circuit in a manner following the frequency change of the horizontal synchronizing signal.
Accordingly, as a consequence of change in the control voltage corresponding to the frequency change of the horizontal synchronizing signal, a steady phase error corresponding to the amount of the change occurs at the phase comparator in this type of AFC circuit when the frequency of the horizontal synchronizing signal is changed.
There is a problem that such steady phase error results in a shift in the horizontal position of a display image, where the image center in the horizontal direction must be readjusted every time when a shift occurs in the horizontal synchronizing frequency.
Further, the steady phase error could be changed by a change in temperature, where the display image would be horizontally shifted in position following an ambient temperature change.
Particularly, in a bi-directional deflecting circuit, if the display images of forward path and backward path are displayed in a manner shifted in the opposite directions from each other due to such steady phase error, a problem occurs that the display image is displayed as an overlapped image or that, if the shift is relatively smaller, resolution is degraded.
In view of the above points, the present invention has been made to propose a deflecting circuit in which a horizontal shift in displayed image-may be prevented even if the horizontal synchronizing frequency is changed when it is used in bi-directional deflection.